Mycobacterium tuberculosis is arguably one of the most successful pathologic micro-organisms worldwide, and is the causative agent of the potentially lethal infectious disease tuberculosis. It is also the leading cause of death worldwide from a potentially curable infectious disease, with an estimated two million related deaths annually.
The pathogenesis of TB is complex, with the initial infection occurring as the result of the inhalation of aerosolized infectious Mycobacterium tuberculosis. The immunological response to the bacillary insult dictates whether the infected individual will proceed to develop either localized pulmonary disease, latent disease (LTBI) or disseminated disease as a consequence of haematogenous spread. Those acquiring latent disease remain asymptomatic but retain the potential to evolve into active disease (this occurs in approximately 10% of all cases over a lifetime period).
It is estimated that approximately one third of the world's population is infected with either latent or active tuberculosis. Recent figures from 2012 report a total of 8751 cases of active tuberculosis in the UK. This represents an overall upward trend over the last decade. The number of Mycobacterium tuberculosisisolates resistant to antibiotics is also increasing.
Pulmonary TB is the most common clinical presentation of infection with Mycobacterium tuberculosis. Symptoms typically include chronic cough with or without haemoptysis, fever, night sweats, and weight loss. Only individuals with active pulmonary disease remain infectious as they have the ability to aerosolize bacilli. Infection of almost any other organ system may occur with diverse accompanying clinical presentations. Immunocompromised individuals may present atypically.
The pathogen has shown a dramatic resurgence, driven in part by the HIV epidemic in sub-Saharan Africa, as the immunological response of these individuals is compromised. This has a two fold impact. Firstly, individuals are more likely to progress from pre-existing latent disease to active disease and secondly, primary infection is more likely to take the form of active disease with increased infectivity. With increasing globalisation, the detection, prevention and early appropriate treatment of this aerosolised pathogen is becoming an increasingly important public health priority.
Despite extensive research, the current understanding of the immunological response and pathogenesis of Mycobacterium tuberculosis remains incomplete. Furthermore, the existing diagnostics and treatment methods are suboptimal. Tuberculosis is definitively diagnosed by the identification of the causative organism (Mycobacterium tuberculosis) in a clinical specimen. This is achieved by prolonged culture of the organism, or by PCR analysis. Adjuncts to definitive diagnosis include: diagnostic imaging (X-rays or radiological scans), tuberculin skin tests (Mantoux/Heaf tests) and Interferon Gamma Release Assays (IGRAs).
Existing barriers to rapid definitive TB diagnosis include the difficulty in culturing this slow-growing organism in the laboratory, which can take around 3 to 12 weeks, or in obtaining an appropriate sample containing Mycobacterial DNA for PCR. The latter may require invasive sampling in the case of extrapulmonary TB, which is costly and may involve additional risks to the patient. Recent developments in the field of TB diagnostics include the Xpert MTB/RIF test which has been endorsed by the World Health Authority This is for use in sputum samples to diagnose cases of suspected active pulmonary tuberculosis, and to detect rifampicin resistant mutations, which are a marker for multi-drug resistant tuberculosis. This methodology is, however, dependent on a PCR-positive sample being obtained.
As a consequence of suboptimal diagnostic tools, anti-microbial combination chemotherapy treatment is often commenced empirically on the basis of clinical suspicion in conjunction with the results of adjunctive diagnostic tests. The rationale for this approach is two fold. The patient receives a therapeutic trial of anti-microbial chemotherapy, and in cases of pulmonary tuberculosis, transmission is curtailed through the reduction of the bacillary load in the sputum.
Current diagnostic adjuncts include: radiological imaging, IGRAs and tuberculin skin testing (TST). Imaging provides guidance as to whether typical features are present, whereas IGRAs and TST provide information regarding possible prior exposure to M. tuberculosis by interrogating the immunological response to TB-related antigens. Interpretation of both the IGRA and TST tests is complex and confounded by a number of factors. These include amongst several others: prior exposure (latent disease), prior vaccination with BCG, and immunosupression. IGRA has, however, increasingly become an accepted adjunctive tool in countries with a low prevalence of TB for evaluating the likelihood of tuberculous disease being present. Unfortunately imaging, TST and the IGRA test are unable to definitively diagnose the presence of active disease.
Recent publications have emphasized the potential for utilising combinations of biomarkers as diagnostic tools for tuberculosis. WO 03/075016 describes that levels of soluble proteins detectable in the blood, namely soluble CD antigens (“sCD”) sCD15, sCD23, sCD27 and sCD54, may be altered in patients with tuberculosis. A Medscape Medical News article from the American Thoracic Society (ATS) 2010 International Conference indicated that a combination of IL-15 and MCP-1 accurately categorised 84% of subjects as having active or latent tuberculosis. Chegou (2nd Global Symposium on IGRAs May-June 2009) describes that combinations of biomarkers are more promising TB diagnostics than individual biomarkers and suggests measurement of EGF, sCD40L, MIP-1β, VEGF, TGF-α or IL-1α as a rapid test for active TB. Wu et al (2007) J Immunol 178, 3688-3694 indicated that IL-8, FOXP3 and IL-12β offer a means of differentiating between latent Mycobacterium tuberculosis infection and active tuberculosis disease.
As current diagnostic methodologies for TB are suboptimal, so too are the treatment options available for this disease. As a consequence of the intracellular nature of this pathogen, and lack of adequate innovations in the field of TB therapeutics, the basis of TB therapy continues to be combination anti-microbial chemotherapy over a prolonged period of months in the simplest cases. Partial compliance with treatment may though lead to suboptimal therapeutic levels of the antimicrobials, and the micro-evolution of antibiotic resistance mutations. As a result of this, patients may remain infectious for longer durations, and the frequency of transmission is enhanced. The development of these resistance mutations, some of which are unresponsive to all known anti TB medications (multi-resistant), has recently caused heightened concern in India.
There is therefore a significant need to identify more effective and efficient methods for definitively diagnosing both active and latent TB and in particular differentiating active from latent TB.